Implement MeleeWeapon class
Work on MeleeWeapon.

Still not working yet. Hit detection for melee attacks always fails.
Problem likely lies in XnaHelper.AngleBetweenVectors.
Implement melee weapon hits
Proper Meleeweapon collision detection
Rework unit data loading from xml
Implement enemy melee weapons.

Enemies can weild melee weapons now. Be afraid!
Specify a melee weapon name in the unit's entry in UnitData.xml and units
of that type will be equipped with that weapon.
Merge pull request #18 from murphyslaw480/master

Melee Weapons and Dangerous Enemies